In the quest for hydrocarbon reservoirs, companies employ many data-gathering techniques, such as well logging. During the well drilling process, or shortly thereafter, drillers pass logging instruments through the borehole to collect information about the surrounding formations. The information is traditionally collected in “log” form, i.e., a table, chart or graph of measured data values as a function of position along the borehole. When the position information for the logging instrument includes both depth and orientation, the log can take the form of a two-dimensional “image” of the borehole wall. Imaging enables analysts to study the fine-scale structure of the penetrated formations, including stratifications, fractures, dip angles, rock texture, vugs, and other features and anomalies. Image logging may be accomplished, for example, by wireline logging or logging-while-drilling (LWD).
In wireline logging, a sonde is lowered into the borehole after some or the entire well has been drilled. The sonde hangs at the end of a long cable (a “wireline”) that provides mechanical support and an electrical connection between the sonde and logging equipment located at the surface of the well. In accordance with existing logging techniques, various parameters of the earth's formations are measured and correlated with the position of the sonde in the borehole as the sonde is pulled uphole. In LWD, the drilling assembly includes sensing instruments that measure various parameters as the formation is being penetrated. While LWD techniques allow more contemporaneous formation measurements, drilling operations create an environment that is generally hostile to electronic instrumentation and sensor operations.
The various types of logging information sought include among others formation density, formation resistivity, acoustic velocity, pore volume, and pore pressure. Formation density is most commonly measured by measuring the attenuation and/or scattering of nuclear radiation from a radioactive source (see, e.g., Well Logging for Earth Scientists, 2d by Darwin Ellis and Julian Singer, © 2007 Springer, ISBN 978-1-4020-3738-2). Accordingly, the acquisition of such logs may require the transport, deployment, and use of radioactive material, with the corresponding allocation of resources for safety and security. Another formation density measurement technique employs a nuclear magnetic resonance (NMR) tool to indirectly estimate formation porosity, which in turn correlates to formation density provided the density of the grain of the matrix is known.
Another measurement of interest in the oil and gas well construction domain is the acoustic impedance at the outer surface of the casing. For well control and zonal isolation requirements there are instances where the outer surface of the casing must be well bonded to a sheath of cement. The interface between the casing and the cement exhibits in that case significantly higher acoustic impedance than if the outer surface of the casing were simply in contact with gas or liquid. The acoustic impedance measurement is then directly interpreted as a cement bond quality log (see, e.g., U.S. Pat. No. 4,255,798, issued Mar. 10, 1981, and R. M. Havira, “Ultrasonic Cement Bond Evaluation”, Paper N, SPWLA Symposium 1982)
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.